1) If I'm constructing a building and its not finished, if I decide to cancel the building do I get my cash back?
2) Do I get cash back after demolishing an already established building?
3) I need more friends add me biggrin *Google+ Players only*
You won't get your cash back. Once bought, you've lost your cash. So if you don't like the building at the moment, just put it in storage? It would be a waste of your gold pastries to simply destroy it.
Btw you can't cancel a building half-way. Once it's there/built/under construction you can't change your mind anymore.